The present invention relates to a device for controlling a motor vehicle to run at a constant speed, and more particularly to a device for controlling a motor vehicle to run at a constant speed while reducing overshoot of vehicle speed at a time of transition when running from an uphill to the running on a flat road.
To maintain a constant speed of travel of a motor vehicle which is continuously running on a flat road or a sloping road, the driver has to adjust the depth to which an accelerator pedal is depressed according to the road conditions. This procedure can be physically and mentally tiresome. There has recently been developed a device, known as a cruise control system, for controlling a motor vehicle to run at a desired constant speed without requiring the driver to depress and adjust the accelerator pedal.
With the conventional constant-speed controlling device, when a large-size motor vehicle of a large inertial mass runs uphill and then on a flat road, as shown in FIG. 8(a) of the accompanying drawings, the actual speed of the motor vehicle overshoots a preset vehicle speed as shown in FIG. 8(c). If such an overshoot is large, the driver and passengers on the vehicle tend to feel uncomfortable, and the motor vehicle tends to shake vertically, causing the load on the motor vehicle to collapse.
The problem of overshooting is particularly experienced by a diesel-engine motor vehicle in which an all-speed governor is mounted on a fuel injection pump. The reason for an increased amount of overshoot on such a motor vehicle in a constant-speed running mode is shown in FIG. 8(c). While running uphill, the control lever of the governor is opened to a full-speed position. A large time lag or delay occurrs before the control lever returns to an opening A corresponding to the preset speed when the vehicle runs on a flat road. In a system in which the opening of a governor control lever is subject to proportional plus integral control (PI control) based on a difference between the preset speed and an actual speed, increased overshooting is caused because the period of integral control is increased and a large time lag elapses until the control lever returns to the opening A. More specifically, as shown in FIG. 9, in order for the motor vehicle to run uphill, the opening B is sufficient as the control lever opening which maintains the engine rotational speed Nb at which an engine torque Tb can be produced, (during uphill running, the transmission is in a lower gear position than the gear position in which the vehicle runs on a flat road with the lever opening A). Therefore, the hatched area shown in FIG. 8(c) indicates wasteful motion of the control lever in the conventional constant-speed controlling device.
One proposed device for controlling a motor vehicle to run at constant speed is disclosed in Japanese Laid-Open Patent Publication No. 60-145430. The disclosed device utilizes auxiliary braking provided by the exhaust as a means for reducing speed overshooting of a large-size motor vehicle having a large inertial mass. The disclosed scheme is however not addressed to the elimination of the wasteful motion of the control lever, but to the solution of the problem of overshooting caused by a control lever being opened to the full-speed position by effecting auxiliary braking provided by the exhaust. Therefore, the problems of excessive fuel consumption and a complex mechanism remain to be solved.